A pair of studies from Tel Aviv University map 110 mass mortality events of sea urchins since 1888 and show that pathogens are the main cause, while also presenting a "corona swab" method for non-invasive underwater genetic sampling to monitor epidemics at sea
A pair of first-of-its-kind studies by researchers from the School of Zoology and the Museum of Natural History at Tel Aviv University, led by Dr. Omri Bronstein, determine the main causes of mass sea urchin deaths in recent decades: pathogens, storms, and extreme temperatures. Dr. Bronstein and his team also developed an innovative method for genetic sampling in marine environments – using a swab and similar to testing for the coronavirus, among other things to enable rapid and non-invasive monitoring of marine animals and the outbreak of underwater epidemics.
The first study published in the journal Biological Reviews Showing A meta-analysis examining all 110 mass mortality events (MMEs) known to science among sea urchins from 1888 to 2024. In this framework, Dr. Bronstein and doctoral student Lisa Schmidt conducted a comprehensive review of the history of these events, which shows that most mass mortality events are reported from the Northern Hemisphere (where most research is conducted and where most research budgets are located), and are concentrated around the United States, Western Europe and Japan. The researchers from Tel Aviv University classified five main causes of these deaths and found that: 33% of the cases were caused by pathogens, 25% by catastrophic events such as storms and oxygen deficiency, 24% by extreme temperatures, 11% by algal blooms and 7% by human activity - such as environmental pollution and destruction of habitats.
Meta – Analysis
"This is a meta-analysis of all the research literature on the subject," says Dr. Bronstein. "For each mass mortality event, we mapped where it occurred, when, in which species the mortality occurred, and most importantly, what the reasons were. It turns out that pathogens are the leading cause of mass mortality among sea urchins. This finding fits in and reliably reflects what we see today in the modern wave of mass mortality - from the Caribbean, through the Red Sea, and to the Indian Ocean. There is a tendency to point to global warming as the cause of everything, but this is not always accurate. In many cases we see that mortality is not directly related to heat, since some of the affected urchin species are naturally able to live in warmer environments. These are not optimal temperatures, but they are not lethal in terms of their upper range either. The problem is that warming affects many other factors in the environment, and these can already constitute a deadly combination. "Thus, in warmer environments we see a decrease in the concentration of dissolved oxygen in the water and increased activity of pathogens, for example."
In this context, it should be noted that in 2023, Dr. Bronstein identified a mass mortality of sea urchins of the long-spined sea urchin species on the shores of the Red Sea. He later identified that the mortality was caused by the same pathogen that wiped out a related species of sea urchin in the Caribbean – a ciliate parasite. Since the initial identification, the epidemic has spread to the Indian Ocean, re-emerged in the Caribbean, and is now defined as a pandemic that threatens sea urchin populations worldwide.
"Sea urchins are essential to the health of the coral reef," explains Dr. Bronstein. "They are the 'gardeners' of the reef: they feed on algae and prevent them from taking over and 'suffocating' the corals that compete with them for sunlight. In 1983, most of the sea urchins in the Caribbean died. The algae there multiplied unchecked, blocking the corals from sunlight, and the entire environment underwent a transformation from coral reefs to seaweed fields. Although 40 years have passed, the sea urchin population in the Caribbean - and with it the reef - has not returned to its previous state. We fear that the same process may now also be realized in other areas of the world where mass mortality is occurring, mainly in the long-spined nun species - a relative of the Caribbean sea urchin. These are the black urchins with the long spines that we all know. Until recently, this was one of the common urchins on the coral reef in Eilat. Today, this sea urchin is almost non-existent in the Red Sea. This is a very violent event: in less than 48 hours, a healthy population of sea urchins turns into decaying skeletons. In some sites in Eilat and Sinai, mortality rates reached 100%. Later, we also saw mass mortality on Reunion Island in the Indian Ocean, and today we are examining three more new cases of mass mortality from the Atlantic, Indian and even Mediterranean Oceans. Thus, a local mass mortality event became regional and then global. This is a danger to coral reefs everywhere in the world."
Following this, and to overcome one of the greatest challenges in collecting genetic information at sea, research student Mai Bonomo and Dr. Omri Bronstein published a separate study in the journal Molecular Ecology Resources, which aims to develop a new, inexpensive, and non-invasive method that allows for the widespread collection of underwater genetic samples.
Identification of pathogens in marine animals
"The main tools used today to identify pathogens are genetic tools," says Dr. Bronstein. "But the field of molecular ecology suffers from a fundamental problem: there is no simple way to sample DNA from underwater animals. Therefore, many studies rely on invasive sampling that causes damage to the animal being sampled and often even requires sacrificing the entire animal and bringing it to the laboratory. Therefore, the research operates under very strict regulation that in each case weighs the scientific value against the values of environmental conservation. For example, it is prohibited to sample animals in sites that are considered marine nature reserves, there are prohibitions and restrictions on shipping samples abroad, such as corals, and in every scientific publication it is necessary to present the official permits for each sample presented in the publication. Our need to find a solution to this bottleneck stemmed from the sea urchin epidemic. "There are two ways to detect sick urchins today: to see with your own eyes that they are sick, and then it's too late - these urchins are dying. Alternatively, you can identify the disease before symptoms appear, using genetic tools, but if that means taking them out of the sea - it doesn't matter whether they are sick or if it turns out they aren't, if we sacrificed them anyway."
"Corona" test kit
To solve the great challenge of marine ecology, researchers from Tel Aviv University have developed a special kit for collecting genetic samples underwater. An affordable, reliable, inexpensive, and easy-to-operate kit – and it has already been put into use by various research groups around the world – mainly in remote or particularly sensitive areas.
"We have developed a new tool for underwater DNA sampling, reminiscent of a corona test. At the end of a dedicated test tube filled with the preservation fluid, there is a membrane that prevents water from entering, and a cap with a clip - like some toothpastes. Just like in corona tests, the researcher passes a swab over the surface of the marine animal without causing it any damage or moving it from its place. There is even no need to sample the mucous membranes as in humans, it is enough to gently pass the swab over them. Next, the swab is inserted into the test tube while puncturing the membrane that prevents water from entering the preservation fluid inside, and the squeeze cap is closed to secure the sample. That's the whole story. A single researcher can thus collect hundreds of samples in a single dive in almost any environmental condition and depth that allows diving. The kit has already been tested in challenging environments, such as in field expeditions to Djibouti and Reunion Island, and the results look very promising: samples that then stood for months without refrigeration until they reached our laboratory were exceptionally preserved and also allowed for sensitive genetic analyses. In a large-scale experiment we conducted with the new method here in the Gulf of Eilat, we were able to collect genetic material from hundreds of skin tags in the Gulf – the group that includes sea urchins, starfish and their friends – within a few months and conduct the most extensive genetic analysis ever done on these species in our area, which led to the discovery of several new species and the redefinition of other species that we did not know existed. This is a simple and elegant solution to one of the most vexing technical problems in the field of marine ecology."
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